1. Field of the Invention
This invention relates to an improvement on an exhaust gas processing apparatus for a paint drying oven.
2. Discussion of the Background
Many exhaust gas processing apparatuses have been developed to purify exhaust gases containing various harmful components generated by incinerators. For instance, the following (1) through (6) have been known as typical examples of the exhaust gas processing apparatuses:
(1) U.S. Pat. No. 4,255,173 discloses an automobile type exhaust gas lead filter. The gas lead filter has a tubular housing with alumina coated metal wire fibers as the filter bed, which is centrally located.
(2) U.S. Pat. No. 4,225,561 discloses a catalytic converter for transforming polluting gases into non-polluting gases. The catalytic converter has an open-ended elongated shell, a metallic grill coated with silver, palladium or mixtures thereof, and a metallic mesh coated with silver, palladium or mixtures thereof.
(3) U.S. Pat. No. 4,492,770 discloses a catalyst for an internal combustion engine. The catalyst has a support, an active phase comprising copper, cerium and/or iron, at least one metal selected from the group consisting of platinum and palladium, and at least one metal selected from the group consisting of iridium and rhodium.
(4) Japanese Unexamined Utility Model Publication (KOKAI) No. 118626/1986 discloses an exhaust gas processing apparatus. The exhaust gas processing apparatus comprises two or three layers, i.e. one or two layers of platinum group element catalyst having heat resistance fibers as the support and one layer of copper-cerium catalyst having ceramic porous substance as the support.
(5) Japanese Unexamined Utility Model Publication (KOKAI) No. 118627/1986 discloses another exhaust gas processing apparatus. The exhaust gas processing apparatus comprises two layers, i.e. a platinum group element catalyst and copper-cerium catalyst.
(6) U.S. Pat. No. 4,661,329 (Japanese Unexamined Patent Publication (KOKAI) No. 146348/1986) discloses an oxidation catalyst. The oxidation catalyst comprises copper-cerium-silver catalyst having ceramic porous substance as the support.
The platinum group element catalyst comprises elements of the platinum group, such as platinum, palladium, iridium, rhodium osmium and ruthenium. The platinum group element catalyst has been employed for the exhaust gas processing apparatus, because the platinum group elements catalyst has been known to have a high dehydrogenation activity and a stable heat resistance property.
The copper-cerium-silver catalyst disclosed in above (6) has been employed as a catalyst for efficiently burning and decomposing aldehydes contained in exhaust gases, which the platinum group element catalyst cannot purify sufficiently, at a low temperature.
However, the present inventors experimentally found that tar-like substances produced by incomplete combustion of harmful components contained in exhaust gases deposited on the supports, increased pressure drop and shortened the lives of the catalysts when the platinum group metal catalyst and copper-cerium-silver catalyst were employed for an exhaust gas processing apparatus simultaneously in order to use their respective advantages and when the exhaust gas processing apparatus was operated at relatively low temperatures.
This invention has been developed to overcome these problems. It is therefore an object of this invention to provide an exhaust gas processing apparatus for a paint drying oven maintaining its exhaust gases purifying abilities at relatively low temperatures and exhibiting pressure drop, caused by the tar-like substances and dusts contained in exhaust gases coming from a paint drying oven, in a lesser degree.
An exhaust gas processing apparatus for a paint drying oven of this invention comprises a reaction container; a first catalyst layer disposed at an upstream side of the reaction container comprising a support a honeycomb substance or a foamed substance, a carrier layer formed on the support and a platinum group element catalyst loaded on the carrier layer; a second catalyst layer disposed downstream of the first catalyst layer comprising a support a pellet substance, a honeycomb substance or a foamed substance and a platinum group element catalyst; and a third catalyst layer disposed downstream of the second catalyst layer comprising a support, a carrier layer formed on the support and a copper-cerium-silver catalyst loaded on the carrier layer. The exhaust gas processing apparatus oxidizes and decomposes harmful components contained in exhaust gases coming from a paint drying oven.
The exhaust gas processing apparatus of this invention performs dehydrogenation and combustion reactions of exhaust gases simultaneously, and decreases harmful components in exhaust gases remarkably.
The dehydrogenation reaction is performed by the platinum group elements catalyst of the first and second catalyst layers. These platinum group element catalysts accelerate the dehydrogenation reaction. The dehydrogenation reaction subtracts hydrogen from organic compounds such as phenols and alcohols having a hydroxyl group, and converts them to aldehydes.
The combustion reaction is performed by the copper-cerium-silver catalyst of the third catalyst layer. This copper-cerium-silver catalyst changes the aldehydes converted on the platinum group elements catalysts of the first and second catalyst layers disposed upstream to the third catalyst layer as well as aldehydes originally contained in exhaust gases to carboxylic acids, and finally to harmless and odorless carbon dioxide gas and water.
The exhaust gas processing apparatus of this invention employs a honeycomb or foamed substance for the support of the first catalyst layer, and a pellet, honeycomb or foamed substance for the support of the second catalyst layer. For the support of the third catalyst layer, the exhaust gas processing apparatus may employ a honeycomb or foamed substance. This arrangement can deposit the tar-like substances and dusts in exhaust gases on the first catalyst layer without depositing them on the second and third catalyst layers, because the honeycomb or foamed substance has the tar-like substances deposited on it in greater amount than a support made of heat resistance fibers does, and because the honeycomb or foamed substance causes less pressure drop. The tar-like substances are produced by processing exhaust gases at relatively low temperatures during the dehydrogenation reaction. Here, the foamed substance means a substance like a polyurethane foam having a plurality of cells, and the honeycomb substance means a substance having a plurality of through holes like the six-sided cells made by bees.
Further, it is preferred to employ a support made of an alumina pellet substance, a honeycomb substance or a foamed substance for the second catalyst layer in order to reduce reaction heat with ease and prevent degradation by heating.
Furthermore, the support of the third catalyst layer is not explicitly defined, however, it is preferred to employ a honeycomb or foamed substance for the support when a pellet substance is employed for the second catalyst layer. This arrangement helps to suppress pressure drop increment, because pressure drop increases when a pellet substance is employed for both of the second and third catalyst layers.
Thus, the exhaust gas processing apparatus of this invention suppresses pressure drop increment and accelerates purifying reactions of harmful components in exhaust gases efficiently.
Further, the present inventors experimentally confirmed that the following arrangements for the supports of the first and second catalyst layers of the exhaust gas processing apparatus of this invention have the tar-like substances deposited more on the first catalyst layer and suppress pressure drop:
The honeycomb substance for the supports of the first and second catalyst layers may have a cell density of 400 cells/inch.sup.2 or less and an apparent surface area of from 8 to 25 cm.sup.2 /c.c. It is more effective that the honeycomb substance has a cell density of 200 cells/inch.sup.2 or less and an apparent surface area of from 8 to 25 cm.sup.2 /c.c.
The foamed substance for the supports of the first and second catalyst layers may be 17-mesh or less foamed substance and have an apparent surface area of from 8 to 25 cm.sup.2 /c.c. When the foamed substance with more than 17-mesh is employed, for instance when a 20-mesh foamed substance is employed, the exhaust gas processing ability decreases and pressure drop tends to increase. The foamed substance serves more for the objects of this invention when it is from 6 to 13-mesh foamed substance and has an apparent surface area of from 8 to 25 cm.sup.2 /c.c.
Furthermore, as for a material for the honeycomb and foamed substances for the supports may be cordierite, mullite, aluminum titanate, alumina, silica, silicon carbide, silicon nitride, zircon, Fe-Cr-Al-Y alloy or Ni-Cr alloy.
Moreover, wash coating may be done first on the supports to form a carrier layer, and then catalysts may be loaded on the carrier layer. Or the catalysts may be loaded on a powdery substance for forming the carrier layer, and then loaded the supports by wash coating done simultaneously with carrier layer forming. Ceramics for forming the carier layer may be alumina, silica, magnesia, zeolite, titania and diatomaceous earth and so on.
Thus, in the exhaust gas processing apparatus of this invention, the first catalyst layer has the tar-like substances and dusts deposited thereon, and the platinum group element catalyst of the first catalyst layer performs the dehydrogenation reaction. Then, the platinum group element catalyst of the second catalyst layer performs a thorough dehydrogenation reaction without causing pressure drop at the second catalyst layer. Finally, the copper-cerium-silver catalyst of the third catalyst layer performs the combustion reaction of the aldehydes to decompose them to harmless and odorless carbon dioxide gas and water without causing pressure drop at the third catalyst layer.
As a result, the following advantages can be obtained from the exhaust gas processing apparatus of this invention:
The exhaust gas processing apparatus of this invention improves energy saving effect by lowering the reaction temperature by from 50.degree. to 100.degree. C. to those of conventional exhaust gas purifying apparatuses and cuts down the running cost while maintaining or even improving the purifying abilities of exhaust gases.
Further, an exhaust gas processing apparatus with a longer life can be provided, because the pressure drop is suppressed by having the tar-like substances associating with the lowered reaction temperature and dusts deposited on the first catalyst layer.
Furthermore, an exhaust gas processing apparatus with a simple construction can be manufactured in a less expensive manufacturing cost, because the exhaust gas processing apparatus of this invention performs the dehydrogenation and oxidation/decomposition reactions simultaneously.
The running cost can be reduced further, because the first catalyst layer is washable and can be cleaned easily. Accordingly, man-power requirement for maintenance is minimized and the catalyst life is extended.
Even initial cost for building facilities can be reduced, because less heat resistant and less expensive materials can be employed for oven inner structural materials for building ovens.